1. Field of the Invention
This invention relates to a vacuum fluorescent color print head for printing paper. Generally, this type of fluorescent color print head includes a red luminous block having a plurality of luminous elements arranged in a main scanning direction and red color filters disposed at light emitting ends of the luminous elements, a green luminous block having a plurality of luminous elements arranged in the main scanning direction and green color filters disposed at light emitting ends of the luminous elements, and a blue luminous block having a plurality of luminous elements arranged in the main scanning direction and blue color filters disposed at light emitting ends of the luminous elements,
2. Description of the Related Art
A fluorescent color print head for use on a photographic printer for forming color images on a photosensitive medium includes three luminous blocks, i.e. an R (red) luminous block, a G (green) luminous block and a B (blue) luminous block as disclosed in U.S. Pat. No. 5,592,205 (corresponding to Japanese Patent Laying-Open Publication H5-92622), for example. Each luminous block has filamentary electrodes acting as cathodes for releasing thermions, control electrodes, and a plurality of strip-like anode electrodes covered by phosphorous objects of a predetermined size arranged at predetermined intervals, all sealed in a vacuum case. Color filters are disposed outside the vacuum case on the paths of light beams radiating from the phosphorous objects. Generally, the phosphorous objects are formed of ZnO:Zn phosphor. As shown in FIG. 10, the light beams radiating from these phosphorous objects have wavelengths in a wide band of the order of 430 nm to 760 nm. Thus, a color print head for emitting light beams in the three RGB primary colors is obtained by using red, green and blue filters as color filters for the respective luminous blocks. Theremion impingement upon the phosphorous objects, i.e. light emission from the phosphorous objects, is controlled by applying a voltage to the strip-like anode electrodes and applying control signals based on image data to the control electrodes.
The above color print head has been used mainly for forming latent images on the photoreceptor drum of an optical printer. It has been proposed in recent years to apply such print head to a digital exposing device, instead of a projection type optical exposing device, for exposing photographic printing paper (hereinafter referred to simply as printing paper). However, it is difficult to achieve color reproducibility comparable to the projection type optical exposing device which has attained technological maturity.